iommu: Unexport iommu_group_get_for_dev()

The function is now only used in IOMMU core code and shouldn't be used
outside of it anyway, so remove the export for it.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200429133712.31431-35-joro@8bytes.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>